They were just not designed for this. All 1HZ will get to boiling temp at 100kph uphill, but that is not over heating. 4th gear at 2500rpm is about all you should be doing.During the summer months, I have problems keeping my water temperature under 215° when I’m climbing long steep grades at highway speeds
You could try a manually operated windscreen washer spraying water onto the radiator with its own water bottle to cool it down.
Water injection into the combustion chamber? Water onto a red hot piston causes pitting, a very bad idea